Superabsorbent polymer having improved absorption rate and absorption under pressure

ABSTRACT

A superabsorbent polymer having improved absorption under pressure and fast absorption rate is obtained by first providing a solution containing carboxylic acid monomers or water soluble salts thereof, and a crosslinking agent. A carbonate blowing agent and a polymerization initiator are added, individually or in combination, to the solution to form a carbonated monomer solution. A polymerization initiator is then added to the carbonated monomer solution which is then polymerized at temperatures ranging from about 0° C. to about 130° C., forming a microcellular hydrogel. The microcellular hydrogel is chopped or ground into gel pieces having a particle diameter ranging from about 0.1 mm to about 5.0 cm. The gel pieces are dried at temperatures ranging from about 85° C. to about 210° C., and are then ground to form a polymer having a particle size of from about 0.05 mm to about 5.0 mm. A mixture is formed from 100 parts by weight of the polymer and about 0.001 to about 30 parts by weight of a surface crosslinking agent. The polymer is reacted with the surface crosslinking agent to crosslink molecular chains existing on a surface of the polymer, forming the superabsorbent polymer.

FIELD OF THE INVENTION

The present invention relates generally to superabsorbent polymersexhibiting improved absorption under pressure and fast absorption rates.This invention particularly relates to providing a surface crosslinkedsuperabsorbent polymer which incorporates a blowing agent.

BACKGROUND OF THE INVENTION

Superabsorbent polymers are water insoluble, hydrogel-forming polymerscapable of absorbing large quantities of aqueous fluids includingsynthetic urine, brines, and biological fluids such as urine, sweat, andblood, while retaining the absorbed fluids under pressure.Hydrogel-forming superabsorbent polymers are useful as absorbents forwater and aqueous body fluids when the polymers are incorporated inabsorbent articles, such as disposable diapers, adult incontinence pads,sanitary napkins, and bandages. Many of the existing superabsorbents areformed from unsaturated carboxylic acid monomers including acrylic acid,methacrylic acid, alkylacrylates, and acrylamides which are renderedwater insoluble by crosslinking.

The degree of crosslinking affects the absorbent capacity and gelstrength of a superabsorbent. Capacity is a measure of the amount offluid which a given amount of superabsorbent polymer will absorb. Gelstrength indicates the tendency of the hydrogel once formed to deformunder an applied stress. Polymers exhibiting inadequate gel strengthwill form a hydrogel which deforms and fills the void space in anabsorbent article, inhibiting absorbent capacity and fluid distributionthroughout the article. Polymers having low absorbent capacity areincapable of absorbing a sufficient amount of the fluid encountered inuse of a diaper or other absorbent article. A polymer having a high gelstrength generally possesses a low absorption capacity, and a polymerhaving a high absorption capacity typically possesses a low absorptionrate because of gel blocking phenomenon or low gel strength afterabsorption.

Another characteristic that a superabsorbent polymer must possess is anacceptable level of extractable, water soluble polymer remaining withinthe superabsorbent. The extractable polymer can leach out of a hydrogelwhen fluids contact the superabsorbent. The extractables that leach outof the superabsorbent apparently lower the absorption speed and capacityof the superabsorbent, resulting in leakage of the fluid from anabsorbent article.

Commercially available superabsorbents generally possess sufficientcapacity, but do not have adequate gel strength, swell rate (i.e.,absorption speed) and absorption under pressure for the absorbentarticles of reduced size and thickness that are now being produced. Asfluff fiber in absorbent products is replaced with greater amounts ofsuperabsorbent polymer, the polymer has to perform the functions of thefluff fiber. The polymer must quickly absorb fluid and transport itthroughout an absorbent article without releasing the stored fluid fromthe swelled hydrogel on exertion of pressure. Accordingly, the swollengel particles cannot impede absorption of additional fluid by forming abarrier, but must maintain their liquid permeability.

In order to improve the absorption speed of superabsorbent polymers,blowing agents have been incorporated into superabsorbents as describedin U.S. Pat. Nos. 5,118,719 and 5,145,713. As the blowing agent isdispersed throughout the monomer solution during polymerization, itreleases carbon dioxide when heated. The porosity of the resultantsuperabsorbent polymer provides more surface area within the polymerparticles, increasing the rate at which fluid is absorbed by thepolymer.

The absorption under pressure of a superabsorbent has been improved bycrosslinking the molecular chains at the surface of the polymer. Surfacecrosslinkage also improves the gel strength of the polymer and reducesthe amount of extractables at the polymer surface. Although capacity isreduced at the polymer surface, the core of the polymer, which has lowercrosslink density, retains its absorbance capacity. Crosslinking at thesurface of the polymer particles provides spacings between the particleswhen swelled, allowing fluid to pass the swelled particles and travelthroughout the absorbent article. Surface crosslinkage, however,frequently reduces the absorption speed of the polymer. While thesematerials possess adequate absorption under pressure, they absorbsignificantly slower than the fluff fiber they are replacing in thinnerpersonal care articles.

The polymers which have been crosslinked at their surface (hereinreferred to as core polymers) are not porous materials like thosedisclosed in U.S. Pat. Nos. 5,118,719 and 5,145,713. Accordingly, thesesuperabsorbents generally exhibit slow rates of absorption. U.S. Pat.Nos. 4,666,983 and 5,140,076 disclose absorbent polymers formed fromreacting an absorbent resin powder having a carboxyl group with acrosslinking agent having at least two functional groups per molecule tocrosslink the surface of the polymer. German Patent No. 4,020,780describes surface crosslinked superabsorbent particles formed by coatingmonomers having acid groups with an alkylene carbonate. U.S. Pat. No.5,229,466 discloses surface crosslinking by treating a water swellablecarboxyl group containing polymer with a solution of anN-(hydroxyalkyl)-beta-(meth)-alanine ester or a polycondensation productthereof. A superabsorbent polymer made from acrylic acid, a watersoluble polysaccharide and a crosslinking monomer having at least twopolymerizable ethylenically unsaturated double bonds per molecule isdescribed in U.S. Pat. No. 5,145,906. Water absorbent resins surfacetreated with a polyquaternary amine are disclosed in U.S. Pat. No.4,824,901.

There is a need for a method of producing a water absorbent resin whichexhibits high absorbency under pressure, high absorption speed and highgel strength.

SUMMARY OF THE INVENTION

In order to satisfy the need for improved superabsorbents having greaterabsorption under pressure and fast absorption rate, one aspect of thepresent invention provides a superabsorbent polymer preparable by theprocess of first providing a solution containing carboxylic acidmonomers or water soluble salts thereof, and a crosslinking agent,herein referred to as the monomer solution. A carbonate blowing agentand a polymerization initiator are added, individually or incombination, to the monomer solution to form a carbonated monomersolution. The carbonated monomer solution is then polymerized attemperatures ranging from about 0° C. to about 130° C., forming amicrocellular hydrogel. The microcellular hydrogel is chopped or groundinto gel pieces having a particle diameter ranging from about 0.1 mm toabout 5.0 cm. The gel pieces are dried at temperatures ranging fromabout 85° C. to about 210° C., and are then ground to form a polymerhaving a particle size of from about 0.05 mm to about 5.0 mm. A mixtureis formed from 100 parts by weight of the polymer and about 0.001 toabout 30 parts by weight of a surface crosslinking agent. The polymer isreacted with the surface crosslinking agent to crosslink molecularchains existing on a surface of the polymer, forming the superabsorbentpolymer.

Another embodiment of the present invention is a method of making asuperabsorbent polymer having improved absorption under pressure andabsorption rate when absorbing aqueous body fluids by the aboveidentified process.

A third embodiment of the invention is a method of improving theabsorption under pressure and absorption rate of a superabsorbentpolymer. The superabsorbent polymer is made by the method as describedabove, and is then exposed to aqueous body fluids, when thesuperabsorbent polymer is under the exertion of pressure. Thesuperabsorbent polymer can be contained within a diaper, an incontinencepad, a sanitary napkin or a bandage.

It is an object of this invention to provide a surface crosslinkedsuperabsorbent polymer which exhibits improved absorption under pressurewhile increasing the absorption rate of the polymer with or without anapplied pressure.

An associated object of the invention is to provide a superabsorbentpolymer having a core with a lower crosslink density as compared to thesurface of the polymer such that the core retains its absorbancecapacity.

Another object of the invention is to provide a superabsorbent polymerin which spacings exist between the polymer particles when swelled,allowing fluid to pass the swelled particles and travel throughout theabsorbent article.

Yet another object of the invention is to provide superabsorbent polymerparticles with better bulk, inter- and intra-particle permeability.

Other objects will be apparent to those skilled in the art from thedisclosure herein.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a hydrogel-forming superabsorbent polymerwhich can be incorporated in absorbent articles such as diapers, adultincontinence pads, sanitary napkins, and bandages. The polymer can beused to effectively replace fluff fiber within these products becausethe polymer quickly absorbs fluid and transports it throughout theproduct without releasing the stored fluid from the swelled hydrogel onexertion of pressure. The swollen gel particles do not impede absorptionof additional fluid by forming a barrier, but maintain their liquidpermeability.

The superabsorbent polymer of the present invention may be prepared byfirst forming a monomer solution containing carboxylic acid monomers orwater soluble salts thereof, and an effective water insolubilizingamount of a crosslinking agent. A carbonate blowing agent and apolymerization initiator are added, individually or in combination, tothe monomer solution to form a carbonated monomer solution. Thecarbonated monomer solution is then polymerized to form a microcellularhydrogel. The microcellular hydrogel is chopped or ground into gelpieces which are then dried and ground to form a core polymer.

After the core polymer is made, it is surface crosslinked to providesuperabsorbent particles having a low crosslink density within theparticle core and a high crosslink density on the surface of theparticle. A mixture is formed from 100 parts by weight of the corepolymer and about 0.001 to about 30 parts by weight of a surfacecrosslinking agent. The core polymer is reacted with the surfacecrosslinking agent to crosslink molecular chains existing on a surfaceof the core polymer, forming the superabsorbent polymer.

More specifically, the core polymer is formed in solution, or in awater-in-oil emulsion containing carboxylic acid containing monomers anda crosslinking agent. An effective microcellular forming amount of thecarbonate blowing agent is added to the monomer solution to form thecarbonated monomer solution.

A polymerization initiator is added to the monomer solution or thecarbonated monomer solution for the purpose of initiating the reactionbetween the monomers and the crosslinking agent. The initiator is eitheradded to the monomer solution immediately before or simultaneously withaddition of the blowing agent, or is added to the carbonated monomersolution immediately after the blowing agent has been added to themonomer solution. The initiator is added within no more than fiveminutes before, or fifteen minutes after the addition of the carbonateblowing agent to the monomer solution. Simultaneous addition of bothinitiator and blowing agent, or addition of initiator after the additionof blowing agent is preferred.

The core polymer is preferably formed from a thin layer of thecarbonated monomer solution, which has been deaerated (purged ofoxygen). The thin layer solution is preferably deaerated and protectedfrom air before polymerization, and after initiating polymerization bythe addition of free radical catalysts or by ionizing radical formingradiation. The polymerization forms an aqueous hydrogel of thecrosslinked, water insoluble core polymer. The polymerization isexothermic, causing the gel temperature to increase from the initialtemperature of the carbonated monomer solution of about 0° C. to 20° C.to temperatures of about 80° C. to 130° C. The aqueous gel, in thepresence of the carbonate blowing agent, develops a microcellularhydrogel as polymerization occurs because decomposition of the carbonateblowing agent upon heating disperses carbon dioxide throughout thehydrogel.

The microcellular structure of the core polymer may appear cloudy(demonstrating relatively small dispersed gas bubbles), opaque (normallyrepresenting somewhat larger gas bubbles or higher quantities of carbondioxide), or foamy. The microcellular gel volume increases range fromabout 1.01 to at least 10.0 times the volume of the carbonated monomersolution, primarily depending upon the concentration of the carbonateblowing agent contained in the carbonated monomer solution.

The microcellular gel is then masticated by chopping, grinding, orotherwise forming gel pieces have particle diameter sizes ranging fromabout 0.1 millimeter to about 5.0 centimeters, preferably about 10millimeters to about 2.0 centimeters. These masticated gel pieces arethen dried at temperatures ranging from about 85° C. to about 210° C. toform a dry superabsorbent core polymer. The core polymer is then groundto a particle size having a diameter of from about 0.05 millimeter toabout 5.0 millimeters. The resultant core polymer as described in U.S.Pat. Nos. 5,154,713 and 5,118,719, which are incorporated herein byreference, exhibits an improved rate of absorption of aqueous fluidswhile essentially retaining the gel strength and capacity ofconventional superabsorbents made without carbonate blowing agents.

After the core polymer is formed, the surface region of the core polymeris crosslinked using a surface crosslinking agent to obtain thesuperabsorbent polymer of the present invention. Surface crosslinking ofthe core polymer improves the absorption under pressure, absorptionrate, and gel strength after absorption while maintaining an acceptableabsorption capacity. The surface crosslinking agents for use in thepresent invention include organic carbonates, polyquaternary amines,multivalent cations and compounds possessing in the molecular unitthereof at least two functional groups capable of reacting with thecarboxyl group of the core polymer. 100 parts by weight of the corepolymer formed as described above are mixed with about 0.01 to about 30parts by weight of a surface crosslinking agent, depending upon theselected core polymer, and the surface region of the core polymer isreacted with the surface crosslinking agent. The reaction can be carriedout during and/or after mixing using a conventional mixer.

The reaction between the core polymer and the surface crosslinking agentmay occur at room temperature as when an aziridine compound is selectedas the surface crosslinker. Heat is preferably applied to promote thereaction although it is not required to effect the reaction in manyinstances. When heat must be applied for the reaction to occur as when apolyhydric alcohol, a polyglycidyl compound, a polyamine compound, or apolyoxazoline compound is used as the surface crosslinking agent, theheat is preferably applied after the core polymer and the surfacecrosslinking agent have been mixed. The temperature of the heattreatment is generally in the range of 40° C. to 250° C., preferably inthe range of 90° C. to 250° C. At temperatures exceeding 250° C., thecore polymer could be subject to thermal deterioration. The mixture canbe heated using conventional dryers or heating ovens.

The materials required to produce the superabsorbent polymers of theinvention are specified below. The core polymer is formed using monomersin an aqueous solution, or monomers dissolved in water and thendispersed in a water-in-oil emulsion. The hydrogel-forming core polymeris normally synthesized from aqueous solutions containing acrylic acid,methacrylic acid, their water soluble salts, and mixtures thereof. Themonomers are preferably dissolved in an aqueous solution containing thecrosslinking agent. The monomer solution contains at least 20 percent byweight total monomer content, preferably about 25 to about 75 percent byweight total monomer most preferably from about 30 to about 60 percentby weight total monomer content.

Any olefinically unsaturated carboxylic acid and carboxylic acidanhydride monomers can be used to form the core polymer of theinvention. Suitable monomers include acrylic acids and their anhydridessuch as acrylic acid, methacrylic acid, ethacrylic acid,alpha-chloroacrylic acid, alphacyanoacrylic acid, and beta-methylacrylic acid. For purposes of the present invention, the term(meth)acrylic acid represents the presence of acrylic acid alone,methacrylic acid alone, any admixture of these acids, and any watersoluble salt of these acids, either alone or in admixture. Theunsaturated carboxylic acid monomers can also include itaconic acid,citraconic acid, maleic acid, fumaric acid, and maleic anhydride. Otherolefinic unsaturated monomers can also be used, such as the sulfonicacid monomers. These monomers can be chosen from, but are not limitedto, vinyl sulfonic acids, allyl sulfonic acids, styrene esters includingsulfoacrylic and methacrylic acid esters such as sulfoethylacrylate,sulfoethylmethacrylate, sulfopropylacrylate, andsulfopropylmethacrylate, and sulfo(meth)acrylamide materials such asacrylamido N-methylene sulfonic acid, acrylamido-N-ethylene sulfonicacid, and 2-acrylamido-2-methylpropane sulfonic acid. Other olefinicallyunsaturated monomers such as acrylamide and methacrylamide are useful informing copolymers which can be crosslinked to form a core polymer.

The monomers are preferably selected from the group consisting ofacrylic acid, the water soluble salts of acrylic acid, methacrylic acid,the water soluble salts of methacrylic acid, and mixtures thereof. Apreferred monomer mixture consists essentially of from 20 weight percentto 40 weight percent (meth)acrylic acid and from 60 weight percent to 80weight percent sodium (meth)acrylate.

The core polymer is made from free acid, partially neutralized monomers,or is partially or completely neutralized either before or afterpolymerization by addition of appropriate base materials, such as sodiumhydroxide, ammonia, and the like. Any suitable basic salt forming cationincluding the alkaline metals, ammonia, ammonium salts, and amines maybe used for the purpose of neutralization.

It is preferred to have a degree of neutralization of the carboxylicacid monomers of at least 50 mole percent and up to about 60 to 80 molepercent. When appropriate, the degree of neutralization can be partiallyaccomplished by the addition of the carbonate blowing agents.

Preferably, the carboxyl group is present in the core polymer in anamount of not less than 0.01 equivalent, based on 100 g of the corepolymer. In the case of a partially neutralized polyacrylic acid, forexample, the proportion of the unneutralized polyacrylic acid ispreferably in the range of 1 to 50 mole percent, preferably 5 to 40 molepercent.

The crosslinking agents that can be used to form the monomer solutioninclude, but are not limited to, compounds having at least twopolymerizable double bonds, compounds having at least one polymerizabledouble bond and at least one functional group reactive with the acidcontaining monomer material, compounds having at least two functionalgroups reactive with the acid containing monomer material, andpolyvalent metal compounds, which metallic cations can form ioniccrosslinkages. Crosslinking agents containing at least two polymerizabledouble bonds include di, tri or polyvinyl compounds such as divinylbenzene and divinyl toluene, di, tri or polyesters of unsaturated monoor poly carboxylic acids with polyols including di or tri acrylic acidesters of polyols such as ethylene glycol, trimethylpropane, glycerine,and polyoxyethylene glycols. Other crosslinking agents can includealkylene bis-acrylamides such as N,N'-methylene-bis-acrylamide, carbamylesters obtained by reacting polyisocyanates with hydroxyl groupcontaining monomers, di, tri or poly allyl esters of polyols, di, tri orpoly allyl esters of polycarboxylic acids such as diallyl phthalate anddiallyl adipate, poly carboxylic acid polyols such as trimethylolpropanetriacrylate, esters of unsaturated mono or poly-carboxylic acids withmonoallyl esters of polyols such as the acrylic acid ester ofpolyethylene glycol monoallyl ether, and di or triallyl amine, and thealkylene glycol diglycidyl ethers.

The crosslinking agent is present in an amount ranging from about 0.005weight percent of the monomer solution to about 2.0 weight percent ofthe monomer solution. Preferably, the crosslinking agent is used in anamount ranging from about 0.10 weight percent to about 1.0 weightpercent based on the weight of the monomer solution.

The most preferred crosslinking agents are bis-acrylamides such asN,N'-methylene bis-acrylamide, the di, tri or polyesters of unsaturatedmono or poly carboxylic acid polyols such as trimethylolpropanetriacrylate, the di or tri glycidyl ethers of polyols such as ethyleneglycol diglycidyl ether, the multi-substituted allyl amines such asdiallyl amine and triallyl amine, or mixtures thereof.

Carbonate blowing agents are added to the monomer solution to form thecarbonated monomer solution. The blowing agent releases carbon dioxidewhen heated while dissolved or dispersed in the carbonated monomersolution. The blowing agent can be any carbonate or bicarbonatecontaining salt or mixed salt, and may include carbon dioxide as a gasor a solid, sodium carbonate, potassium carbonate, ammonium carbonate,magnesium carbonate, or magnesium (hydroxic) carbonates, calciumcarbonate, barium carbonate, bicarbonates and their hydrates, or othercations, as well as naturally occurring carbonates, such as dolomite,and mixtures thereof. Preferred carbonate blowing agents are MgCO₃,(NH₄)₂ CO₃, Na₂ CO₃, and mixtures thereof.

It is preferred to add from about 0.05 to about 2.5 weight percentblowing agent based on the weight of the carbonated monomer solution. Itis most preferred to add from about 0.2 weight percent to about 2.5weight percent blowing agent. The blowing agents must be added before orimmediately after polymerization is initiated. The blowing agents arenot effective if added after the hydrogel is formed, nor is it effectivewhen added after chopping or drying the gelled polymer.

The preferred blowing agents are carbonate salts of multivalent cationssuch as magnesium, calcium, and zinc. Although many of the multivalenttransition metal cations can be used, some of them, such as ferriccation, can cause color staining and may be subject toreduction-oxidation reactions or hydrolysis equilibria in water. Thismay lead to difficulties in quality control of the final polymericproduct. Also, other multi-valent cations such as nickel, barium,cadmium, and mercury would be unacceptable because of potential toxic orskin sensitizing effects.

The polymerization of the carbonated monomer solution is initiated withfree radical catalysts which are well known in the art. These initiatorsmay include, but are not limited to, peroxide or persulfate catalysts,azo catalysts, the so called redox catalysts, or any combinationthereof. Preferred catalysts include hydrogen peroxide, sodiumpersulfate, organic peroxides, sodium bisulfite, peracetate catalysts,azo catalysts and mixtures thereof.

The surface crosslinking agents for use in the present invention includeorganic carbonates, polyquaternary amines, polyvalent metal compoundsand compounds possessing in the molecular unit thereof at least twofunctional groups capable of reacting with the carboxyl group of thecore polymer. The surface crosslinking agents include polyhydricalcohols such as ethylene glycol, diethylene glycol, triethylene glycol.tetraethylene glycol, polyethylene glycol, glycerin, polyglycerol,propylene glycol, diethanolamine, triethanolamine, polyoxypropylene,oxyethylene-oxypropylene block copolymer, sorbitan fatty acid esters,polyoxyethylene sorbitan fatty acid esters, trimethylolpropane,pentaerythritol, polyvinyl alcohol and sorbitol, polyglycidyl ethercompounds such as ethylene glycol diglycidyl ether, polyethylene glycoldiglycidyl ether, glycerin diglycidyl ether, glycerol polyglycidylether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether,sorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether,propylene glycol diglycidyl ether, and polypropylene glycol diglycidylether, polyaziridine compounds such as2,2-bishydroxymethylbutanol-tris[3-(1-aziridinyl)propionate],1,6-hexamethylene diethylene urea, anddiphenylmethane-bis-4,4'-N,N'-diethylene urea, haloepoxy compounds suchas epichlorohydrin and α-methylfluorohydrin, polyamine compounds such asethylene diamine, diethylene triamine, triethylene tetramine,tetraethylene pentamine, pentaethylene hexamine, and polyethylene imine,polyisocyanate compounds such as 2,4-tolylene diisocyanate andhexamethylene diisocyanate, hydroxides of zinc, calcium, magnesium,aluminum, iron, titanium and zirconium, halogenides, alkylene carbonatessuch as 1,3-dioxolan-2-one and 4-methyl-1,3-dioxolan-2-one, polyvalentmetal compounds such as salts, (represented by sulfates), for example,and polyquaternary amines such as condensation products of dimethylamineand epichlorohydrin, homo and copolymers of diallyldimethyl ammoniumchloride, and homo and copolymers of dimethylaminoethyl(meth)acrylatemethyl chloride quaternary ammonium salts. One crosslinking agent or twoor more mutually unreactive crosslinking agents selected from the groupmentioned above are used.

Preferably, the surface crosslinking agent includes at least onecompound selected from the group consisting of diethylene glycol,triethylene glycol, polyethylene glycol, glycerin, polyglycerin,propylene glycol, diethanolamine, triethanolamine, polyoxypropylene,oxyethylene-oxypropylene block copolymer, sorbitan fatty acid esters,polyoxyethylene sorbitan fatty acid esters, trimethylol propane,pentaerythritol, polyvinyl alcohol, sorbitol, ethylene carbonate, andethylene glycol diglycidyl ether. The crosslinking agent is used in anamount ranging from about 0.01 to about 30 parts by weight, preferablyabout 0.1 to about 10 parts by weight, based upon 100 parts by weight ofthe core polymer.

Organic solvents or water can be added as the core polymer and thesurface crosslinking agent are mixed to promote uniform dispersion. Theamount of water to be used is in the range of 0 to about 50 parts byweight, preferably up to about 40 parts by weight, more preferably up toabout 20 parts by weight, based upon 100 parts by weight of the corepolymer. If the amount of water exceeds 50 parts by weight, drying timeis increased and the surface crosslinking agent permeates to the centerof the core polymer particles, reducing the absorption capacity.

Organic solvents can be added to the mixture in an amount of 0 to about60 parts by weight, preferably up to about 10 parts by weight, basedupon 100 parts by weight of the core polymer. Amounts of solvent inexcess of 60 parts by weight increase the required drying time of thesuperabsorbent. The organic solvents include lower alcohols such asmethanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol,secbutanol, and t-butanol, ketones such as acetone, methylethyl ketoneand methylisobutyl ketone, ethers such as dioxane, tetrahydrofuran anddiethyl ether, amides such as N,N-dimethyl formamide and N,N-diethylformamide, and sulfoxides such as dimethyl sulfoxide.

Absorbent articles such as diapers, adult incontinence pads, sanitarynapkins and bandages may have a core entirely composed of thesuperabsorbent polymer of the present invention, or can include polymerlayers comprised of up to 100% of the superabsorbent polymer within thecore material.

The following examples are presented to describe preferred embodimentsand utilities of the present invention and are not meant to limit thepresent invention unless otherwise stated in the claims appended hereto.

EXAMPLE 1

The following test methods were performed to determine thecharacteristics of the tested polymers. Absorption under pressure (AUP)is determined by first placing a glass fritted plate inside acrystallizing dish. The dish is filled with 0.9 wt. % saline orsynthetic urine to the top level of the filter plate and a piece ofWhatman filter paper #1 is placed on top of the plate. 0.9000 grams ofpolymer is distributed evenly over a 400 mesh screen fixed to the bottomof a plastic cylinder. A teflon plunger and weight exerting 0.3 or 0.7psi is placed inside the cylinder and the entire cylinder assembly isweighed. The cylinder assembly is placed on the filter plate and allowedto absorb fluid for a specified time and then reweighed. The level offluid in the dish is kept constant during absorption. The absorptionunder pressure is calculated by dividing the difference in the completecylinder apparatus weight before and after absorption by the dry weightof the polymer.

Centrifuge capacity is calculated by placing 0.2000±0.005 grams ofpolymer in a tea bag and sealing the bag. The bag is then placed in 0.9%saline or synthetic urine along with blanks (empty bags) and is allowedto absorb for a specified time. The tea bags are removed from the testliquid and centrifuged at 1500 rpm for three minutes. The weight of thetea bags and blanks is recorded. The absorptive centrifuge capacity iscalculated by subtracting the weight of the blank and the dry polymerfrom the weight of the wet bag containing polymer, and dividing thatamount by the dry polymer weight.

Swell rate is determined by screening 0.358±0.001 grams polymer to a-20/+50 mesh particle size, weighing the polymer and placing it in a 16mm test tube. 10.0 ml of synthetic urine is added to the tube at timezero. The time required for the gel to swell to the bottom of the fluidmeniscus is recorded. The swell rate is calculated by dividing thevolume of fluid absorbed by the mass of the polymer used and the timerequired to absorb the fluid.

The synthetic urine tested contains the following cations and anions orelements dissolved in water: 600-700 ppm Na⁺, 65-75 ppm Ca²⁺, 55-65 ppmMg²⁺, 1100-1200 ppm K⁺, 240-280 ppm phosphorus, 450-500 ppm sulfur,1100-1300 ppm chloride, and 1300-1400 ppm sulfate.

Aqueous monomer solutions containing about 30 to 40 weight percent of acombination of acrylic acid and sodium acrylate in a ratio of about30:70 and triallyl amine crosslinking agent in the amounts specified inTable 1 were formed. The samples were polymerized without incorporationof a blowing agent. The resultant gel was ground and dried, and thecharacteristics of the nonporous core polymer were measured using thetests described above for determining gel strength, capacity and swellrate. These results are reported as samples 1 and 3 in Table 1. Thenonporous core polymers of samples 1 and 3 were then surface crosslinkedby reacting the polymers with from about 1 to about 3 weight percentethylene carbonate to form the superabsorbent polymers of samples 2 and4, respectively. The gel strength, capacity and swell rate for thesamples are listed below:

                                      TABLE 1                                     __________________________________________________________________________                                AUP   AUP                                                 Swell Rate                                                                           Gel Strength                                                                         Capacity                                                                            @ 5 mins.                                                                           @ 60 mins.                                  Sample  (g/g sec)                                                                            (dynes/cm.sup.2)                                                                     (g/g) (g/g) (g/g)                                       __________________________________________________________________________    1: 900 ppm                                                                            0.21   33,535 47.9   4.3 (.3 psi)                                                                        8.4 (.3 psi)                               crosslinker, no              4.1 (.7 psi)                                                                        7.7 (.7 psi)                               blowing agent                                                                 2: surface                                                                            0.17   55,400 39.7  26.1 (.3 psi)                                                                       33.1 (.3 psi)                               crosslinked                 17.0 (.7 psi)                                                                       20.4 (.7 psi)                               sample 1                                                                      3: 2500 ppm                                                                           0.28   77,310 35.6   6.5 (.3 psi)                                                                       22.4 (.3 psi)                               crosslinker, no              4.4 (.7 psi)                                                                        9.0 (.7 psi)                               blowing agent                                                                 4: surface                                                                            0.23   99,100 30.0  29.6 (.3 psi)                                                                       34.9 (.3 psi)                               crosslinked                 24.7 (.7 psi)                                                                       28.9 (.7 psi)                               Sample 3                                                                      __________________________________________________________________________

After the surfaces of the nonporous core polymers were crosslinked, theswell rate and capacity were reduced while the gel strength andabsorption under pressure substantially improved.

EXAMPLE 2

Aqueous monomer solutions containing about 30 to 40 weight percent of acombination of acrylic acid and sodium acrylate in a ratio of about30:70 and triallyl amine crosslinking agent in the amounts specified inTable 2 were formed. The samples were polymerized without incorporationof a blowing agent. The resultant gel was ground and dried, and thecharacteristics of the nonporous core polymer were measured using thetests described above for determining gel strength, capacity and swellrate. These results are reported as samples 1, 3 and 5 in Table 2. Thenonporous core polymers of samples 1, 3 and 5 were then surfacecrosslinked by reacting the polymers with from about 3 weight percentglycerol to form the superabsorbent polymers of samples 2, 4 and 6,respectively. The gel strength and swell rate for the samples are listedbelow:

                  TABLE 2                                                         ______________________________________                                                           Swell Rate Gel Strength                                    Sample             (g/g sec)  (dynes/cm.sup.2)                                ______________________________________                                        1: 900 ppm crosslinker, no blowing                                                               0.20       60,360                                          agent                                                                         2: Surface crosslinked Sample 1                                                                  0.27       58,815                                          3: 1500 ppm crosslinker, no blowing                                                              0.21       62,910                                          agent                                                                         4: Surface crosslinked Sample 3                                                                  0.33       66,785                                          5: 1500 ppm crosslinker, no blowing                                                              0.27       67,230                                          agent                                                                         6: Surface crosslinked Sample 5                                                                  0.39       69,015                                          ______________________________________                                    

After the surfaces of the nonporous core polymers were crosslinked, theswell rate unexpectly improved, and the gel strength improved.

EXAMPLE 3

Aqueous monomer solutions containing about 30 to 40 weight percent of acombination of acrylic acid and sodium acrylate in a ratio of about30:70 and triallyl amine crosslinking agent in the amounts specified inTable 3 were formed. 0.4 weight percent Na₂ CO₃ blowing agent was addedto the monomer solution, forming a carbonated monomer solution. Thesamples were polymerized and the resultant microcellular gel was groundand dried. The characteristics of the porous core polymer were measuredusing the tests described above for determining gel strength, capacityand swell rate. These results are reported as samples 1, 3 and 5 inTable 3. The porous core polymers of samples 1, 3 and 5 were thensurface crosslinked by reacting the polymers with from about 1 to about3 weight percent glycerol to form the superabsorbent polymers of samples2, 4 and 6, respectively. The gel strength, capacity and swell rate forthe samples are listed below:

                                      TABLE 3                                     __________________________________________________________________________                                AUP   AUP                                                 Swell Rate                                                                           Gel Strength                                                                         Capacity                                                                            @ 5 mins.                                                                           @ 60 mins.                                  Sample  (g/g sec)                                                                            (dynes/cm.sup.2)                                                                     (g/g) (g/g) (g/g)                                       __________________________________________________________________________    1: 1,400 ppm                                                                          0.49   48,845 42.5   4.8 (.3 psi)                                                                       10.2 (.3 psi)                               crosslinker,                 4.0 (.7 psi)                                                                        8.0 (.7 psi)                               blowing agent                                                                 2: Surface                                                                            0.57   69,760 35.5  30.6 (.3 psi)                                                                       37.5 (.3 psi)                               crosslinked                 11.4 (.7 psi)                                                                       20.4 (.7 psi)                               Sample 1                                                                      3: 2,000 ppm                                                                          0.47   67,470 39.1   5.1 (.3 psi)                                                                       12.9 (.3 psi)                               crosslinker,                 4.1 (.7 psi)                                                                        8.2 (.7 psi)                               blowing agent                                                                 4: Surface                                                                            0.58   79,245 33.1  33.8 (.3 psi)                                                                       36.7 (.3 psi)                               crosslinked                 21.0 (.7 psi)                                                                       29.4 (.7 psi)                               Sample 3                                                                      5: 2,500 ppm                                                                          0.51   76,785 35.9   6.4 (.3 psi)                                                                       18.1 (.3 psi)                               crosslinker,                 4.1 (.7 psi)                                                                        8.4 (.7 psi)                               blowing agent                                                                 6: Surface                                                                            0.67   95,755 30.1  33.4 (.3 psi)                                                                       34.9 (.3 psi)                               crosslinked                 23.3 (.7 psi)                                                                       28.7 (.7 psi)                               Sample 5                                                                      __________________________________________________________________________

After the surfaces of the porous core polymers were crosslinked, theswell rate, gel strength and absorption under pressure substantiallyimproved. Although the capacity was reduced, it remained within anacceptable range. When the characteristics of the superabsorbentsderived from the porous core polymers are compared to those derived fromthe nonporous core polymers of Example 1, it is evident thatincorporation of a blowing agent in the core polymer improved the swellrate of the polymer even though the swell rate, in most instances, wouldhave declined after surface crosslinking. When compared to the nonporouscore polymers of Example 2, the swell rates of the superabsorbentsderived from porous core polymers were two to three times greater thanthose of Example 2.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example and were herein described in detail. It should be understood,however, that it is not intended to limit the invention to theparticular forms disclosed, but on the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

We claim:
 1. A method of making a superabsorbent polymer having improvedabsorption under pressure and improved absorption rate when absorbingaqueous body fluids, wherein the method comprises the steps of:(a)providing a solution containing carboxylic acid monomers or watersoluble salts thereof, and a crosslinking agent; (b) adding a carbonateblowing agent and a polymerization initiator, individually or incombination, to the solution to form a carbonated monomer solution; (c)polymerizing the carbonated monomer solution at temperatures rangingfrom about 0° C. to about 130° C. to form a microcellular hydrogel; (d)chopping or grinding the microcellular hydrogel into gel pieces having aparticle diameter ranging from about 0.1 mm to about 5.0 cm; (e) dryingthe gel pieces at temperatures ranging from about 85° C. to about 210°C.; (f) grinding the pieces to form a polymer having a particle size offrom about 0.05 mm to about 5.0 mm; (g) mixing 100 parts by weight ofthe polymer with about 0.001 to about 30 parts by weight of a surfacecrosslinking agent; and (h) reacting the polymer with the surfacecrosslinking agent to crosslink molecular chains existing on a surfaceof the polymer, forming the superabsorbent polymer.
 2. The method ofclaim 1 wherein the carbonated monomer solution of step (b) is anaqueous solution containing from about 20 wt. % to about 40 wt. %(meth)acrylic acid monomers consisting essentially of from 20 wt. % to40 wt. % (meth)acrylic acid and from 60 wt. % to 80 wt. % sodium(meth)acrylate, from about 0.05 wt. % to about 2.5 wt. % blowing agentand from about 0.005 wt. % to about 2.0 wt. % crosslinking agent.
 3. Themethod of claim 2 wherein the blowing agent of step (b) is a carbonatecontaining salt, a bicarbonate containing salt, or gaseous or solidcarbon dioxide.
 4. The method of claim 3 wherein the blowing agent isselected from the group consisting of CO₂, Na₂ CO₃, K₂ CO₃, (NH₄)₂ CO₃,MgCO₃, (MgCO₃)₄.Mg(OH)₂.5H₂ O, CaCO₃, ZnCO₃, and mixtures thereof. 5.The method of claim 2 wherein the crosslinking agent of step (a) has atleast two polymerizable double bonds, at least one polymerizable doublebond and at least one functional group reactive with the acid monomersor the water soluble salts thereof, at least two functional groupsreactive with the acid monomers or the water soluble salts thereof, oris a polyvalent metal compound.
 6. The method of claim 5 wherein thecrosslinking agent is a bis-acrylamide, a di, tri or polyester of anunsaturated mono or poly carboxylic acid polyol, a di or tri glycidylether of a polyol, a multi-substituted allyl amine or mixtures thereof.7. The method of claim 1 wherein the polymerization initiator of step(b) is selected from the group consisting of hydrogen peroxide, sodiumpersulfate, azo catalysts, organic peroxides, sodium bisulfate,peracetate catalysts and mixtures thereof.
 8. The method of claim 1wherein the carboxylic acid monomers of step a are selected from thegroup consisting of acrylic acid, methacrylic acid, acrylamide,methacrylamide, ethacrylic acid, alpha-chloroacrylic acid,alpha-cyanoacrylic acid, beta-methylacrylic acid, itaconic acid,citraconic acid, maleic acid, fumaric acid, maleic anhydride, vinylsulfonic acids, allyl sulfonic acids, sulfoethylacrylate,sulfoethylmethacrylate, sulfopropylacrylate, sulfopropylmethacrylate,acrylamido N-methylene sulfonic acid, acrylamido-N-ethylene sulfonicacid, 2-acrylamido-2-methylpropane sulfonic acid, acrylamide,methacrylamide and mixtures thereof.
 9. The method of claim 1 furtherincluding the step of:(i) drying the superabsorbent polymer byapplication of heat.
 10. The method of claim 1 wherein the surfacecrosslinking agent of step (g) is an organic carbonate, a polyvalentmetal, a polyquaternary amine or a compound having at least twofunctional groups, per molecular unit, capable of reacting with acarboxyl group of the polymer.
 11. The method of claim 10 wherein thesurface crosslinking agent is diethylene glycol, triethylene glycol,polyethylene glycol, glycerin, polyglycerin, propylene glycol,diethanolamine, triethanolamine, polyoxypropylene,oxyethylene-oxypropylene block copolymer, sorbitan fatty acid ester,polyoxyethylene sorbitan fatty acid ester, trimethylol propane,pentaerythirtol, sorbitol, diglycidyl ether, a polyvalent metal,alkylene carbonate, a polyquaternary amine or mixtures thereof.
 12. Themethod of claim 1 wherein the carbonate blowing agent is added to themonomer solution no more than five minutes before the initiator isadded.
 13. The method of claim 1 wherein the initiator is added to themonomer solution no more than fifteen minutes after the carbonateblowing agent is added.
 14. A method of improving the absorption underpressure of a superabsorbent polymer, the method comprising the stepsof:(a) providing a solution containing carboxylic acid monomers or watersoluble salts thereof, and a crosslinking agent; (b) adding a carbonateblowing agent and a polymerization initiator, individually or incombination, to the solution to form a carbonated monomer solution; (c)polymerizing the carbonated monomer solution at temperatures rangingfrom about 0° C. to about 130° C. to form a microcellular hydrogel; (d)chopping or grinding the microcellular hydrogel into gel pieces having aparticle diameter ranging from about 0.1 mm to about 5.0 cm; (e) dryingthe gel pieces at temperatures ranging from about 85° C. to about 210°C.; (f) grinding the pieces to form a polymer having a particle size offrom about 0.05 mm to about 5.0 mm; (g) mixing 100 parts by weight ofthe polymer with about 0.001 to about 30 parts by weight of a surfacecrosslinking agent; (h) reacting the polymer with the surfacecrosslinking agent to crosslinking molecular chains existing on asurface of the polymer, forming the superabsorbent polymer; and (i)exposing the superabsorbent polymer to aqueous body fluids, thesuperabsorbent polymer being under exertion of pressure.
 15. The methodof claim 14 wherein the superabsorbent polymer is contained within adiaper, an incontinence pad, a sanitary napkin or a bandage when thesuperabsorbent polymer is under the exertion of pressure.